Measuring ingredients: volume vs weight

I don’t know about you, but one of the first things I noticed when I got into DIY body and skincare was how cosmetic recipes tended to list and measure their ingredients – both solids and liquids – according to mass (or weight in everyday usage) and not by volume. In a cooking recipe, it is not uncommon to see ingredient amounts given as “500ml of vegetable stock”, “2 cups of rice” or even “3 tablespoons of sugar”. In the majority of cosmetic recipes I’ve come across, however, ingredients are measured in grams (or ounces, if I’m looking at an North American recipe); not many recipes call for “1/4 cup of beeswax”, “1 tablespoon of cocoa butter”, or “1/4 teaspoon of natural vitamin E”. And, even though my carrier oils all come in 100, 250 or 500ml bottles, recipes using oils are more likely to ask for “14g” rather than “14ml”, “0.5 fl oz” or “1 tablespoon”.

So I started looking into the differences between measuring by volume and weight* to understand why the latter is preferred in cosmetics formulas and recipes – and I will now try to articulate what I learned!

[*From now on, I’ll be using the everyday term weight, rather than the scientifically correct term mass, to describe how heavy something is.]

The way I see it, the issue of volume vs weight can be tackled from a number of perspectives:

In practical terms

When writing recipes, many people consider it good practice to use a standard unit of measurement that can be applied to both solids and liquids, simply for the sake of consistency and ease of reading. And since it’s easier to measure a liquid by weight than it is to measure a solid by volume (see next point for more), it makes sense to use weight as a unit of measurement.

Measuring solids by weight rather than volume is generally regarded as more accurate, not only because it can be tricky to get certain solids to fit “snugly” in a container to measure their volume (especially if they are awkwardly shaped), but also because the amount of space a solid substance occupies inside a container (and the amount of empty space there is) can vary depending on the shape of the solid.

This example is junior science, really, but it illustrates the concept really well: Which of these cups holds more shea butter?

You can see for yourselves the difference in weight between the two.

The difference may be small here, but you get the idea!

Another reason why measuring by weight is preferable to measuring by volume is that you can be more precise with the former. For example, it’s relatively straightforward to measure out exactly 13.82g of apricot kernel oil (using a scale), but it won’t be so easy to measure out a heaped tablespoon of the same stuff. (Besides, your definition of a “heaped”tablespoon may be different from mine!)

It’s a lot easier to find a scale that can measure a large range of weights in the same unit than it is to find an everyday tool capable of covering a similar range of volumes on a similar scale. My digital kitchen scale can accurately measure objects weighing between 0.1g and 2000g – yet I can’t think of an everyday tool that can cover measurements from 0.1ml to 2000ml! This is why, for DIYers and professional cosmetic formulators alike, it is just easier to follow and develop recipes in which all the ingredient amounts can be accurately and easily worked out, in a consistent unit of measurement, using one piece of equipment: the kitchen scale.

The scientific angle

(To all you physicists out there, please let me know if what I’m saying here is incorrect, or if I’ve missed anything important!)

When it comes to measuring solids and liquids, weight is generally considered as more accurate than volume. This is because the weight of a substance on Earth will remain constant even if its surrounding condition changes (e.g. if it gets hotter or colder). On the other hand, the amount of space (volume) occupied by a substance – especially if it is a liquid – can be affected by things like changes in temperature without the substance changing its form (i.e. whilst remaining a liquid still).

Take a thermometer for example: when it gets warmer, the mercury inside the glass tube expands and rises – thus occupying more space in the tube. [Image credit: https://flic.kr/p/iK8xJs, by Da Sal, reproduced under CC BY 2.0]

Furthermore, different liquids have different densities, which means there isn’t a standard weight per volume unit that can be applied to all liquids. Take the following for example:

1 metric cup of generic bottled water = 250.44g

1 metric cup of apricot kernel oil = 230.36g

1 metric cup of olive oil = 228.24g

The differences may not seem that big or problematic when you’re dealing with small amounts, but when making something in large batches, such differences become much more pronounced. More importantly, they will affect the overall ratio of ingredients in a recipe. For example, if you try to substitute a cup of olive oil with a cup of apricot kernel oil in a recipe, the overall weight of your mixture will increase by 2.12g – this means you will need to adjust your other ingredient amounts so that the overall formula still adds up to 100 per cent.

What the industry regulations say

Ultimately – and this is perhaps most relevant to our discussion here – it seems that for a solid cosmetic product, or a product containing a mixture of solids and liquids, weight is a preferred indicator of the ingredients because it is a regulatory requirement for commercial cosmetic producers:

The EU Cosmetics Regulation (EC) No. 1223/2009 states that the nominal content of a product (over 5g or 5ml) may “be given either by weight or by volume”, though the list of ingredients of a product “shall be established in descending order of weight of the ingredients at the time they are added to the cosmetic product”. Furthermore, the UK Weights and Measures (Packaged Goods) Regulation (2006) states: “Where a package contains liquid its nominal quantity must be indicated by the volume at 20°C. Where it contains a product other than liquid, including a mixture of liquid and solids, its nominal quantity must be indicated by weight. As a general guide, a liquid is normally considered to be a product that, at 20°C, will easily pour and can be measured in a measuring cylinder.”

According to the cosmetic labeling guide by US Food and Drug Administration (FDA), “[unless] there is a firmly established, general consumer usage or trade custom to the contrary, the statement must be in terms of fluid measure if the cosmetic is liquid and in terms of weight if the cosmetic is solid, semi-solid, viscous, or a mixture of solid and liquid. Fluid measures must express the volume at 68°F (20°C).”

So there you have it: weight is preferable to volume when it comes to measuring cosmetic ingredients because (a) it is more practical, (b) more accurate, and (c) it is a regulatory requirement across the cosmetic industry. I hope what I’ve said so far makes sense and I haven’t bored you to death yet … ?

Volume-to-weight conversion for carrier oils

Of course, the reality is that we’re still likely to come across cosmetic recipes using volume as a unit of measurement for oils. So, whenever I follow a recipe and need to convert volumes to weights, I turn to Aqua Calc. This is a volume-to-weight converter based on information provided by the United States Department of Agriculture (USDA) National Nutrient Database for Standard Reference.

An exemption: essential oils

Many people (including myself) often like to add essential oils to a product. However, because different essential oils have different densities, and because we only use a tiny amount each time, it is really tricky to measure a few drops of essential oil by weight. This is why in many recipes you will see “drops” used as a unit of measurement for essential oils! It is possible to convert the drops into grams or dry ounces, by converting the drops first into millilitres or fluid ounces, then from ml or fl. oz. into grams or ounces*. I found a really detailed step-by-step guide from Mountain Rose Blog on how to do this. [*Note: there is a difference between a fluid ounce and an ounce! The former is a unit of volume, the latter a unit of weight.].

If you’re still reading this, then thank you for staying with me to the end ?. Please let me know if there is anything you’re not sure or would like to know more! My research in this area is by no means definitive (below are some links containing further information and guidelines by the professionals!), but I hope I’ve managed to at least shed some light on the question of volume vs weight!

Additional reading

The Soap Queen website has a good article on comparing volume and weight of a variety of ingredients.